As a method for producing a styrene resin foam, a method for producing such foam continuously by plasticizing a styrene resin in an extruder or the like, introducing a volatile organic blowing agent such as methyl chloride into the resin to form a mixture, cooling the resultant mixture to a temperature suitable for obtaining a satisfactory foam, and then extruding the mixture into a lower pressure zone is proposed in JP, A, 47-9593, 52-17574 and 52-94366.
Such known methods propose methyl chloride as a volatile blowing agent suitable in producing satisfactory foams. Methyl chloride has been considered to be an extremely suitable blowing agent for the following reasons: Methyl chloride has a high ability of plasticizing a styrene resin and allows the production of a foam to be performed under a reduced extrusion pressure whereby contributing greatly to an industrially advantageous method for producing a styrene resin foam. Further, methyl chloride has extremely high permeability through a styrene resin foam so that it hardly remains in the foam whereby achieving a dimensional stability of the foam. Accordingly it has been employed for a long period.
Recently, a desire for paying much attention and taking measures in handling methyl chloride is increasing, and from the viewpoint of environmental compatibility, a substitute for methyl chloride is desirable if the performance of the foam industrially required can be maintained.
On the other hand, the so-called hydrocarbons and the so called flons are also proposed as examples of other volatile blowing agents (or blowing agents, when referred simply), and some are used industrially.
For example, a combination of isobutane (i-butane) and normal butane (n-butane) which are less permeable through styrene resin is employed as a blowing agent to obtain a foam having an excellent heat insulating property, as disclosed in JP, A, 1-174540. Alternatively, since an agent such as butane serves to provide a heat insulating property when allowed to remain in a certain amount in a foam, it may be employed in combination with methyl chloride to obtain a satisfactory foam, as disclosed in JP, A, 51-92871.
Among flons, a chlorine atom-containing halogenated carbon (hereinafter referred to as CFC) which is less toxic, nonflammable and chemically stable is proposed to be employed, as disclosed in JP, B, 41-672. Generally, flons have a tendency of remaining in a foam and a low thermal conductivity, which may contribute to the heat insulating property of a foam besides the ability of producing a satisfactory foam. Accordingly, flons tend to be considered to be essential for achieving a high heat insulating property.
However, it is recently pointed out that CFCs have some adverse effects on the ozone layer, and are desired to be replaced if possible with any substitute.
Under such circumstances, a variety of attempts have been made to achieve a satisfactory environmental compatibility.
First, as a substitute of alkyl chlorides represented by methyl chloride, ethers or inorganic gasses such as carbon dioxide are proposed or investigated.
For example, JP, A, 7-507087 discloses to obtain a styrene resin extruded foam body having a thickness of 20 mm or more and a cross-sectional area of 50 cm.sup.2 or more by using a mixture of dimethyl ether and carbon dioxide in a specific mixing range. In addition to dimethyl ether and carbon dioxide, various substances such as saturated hydrocarbons, chlorinated fluorinated hydrocarbons obtained by replacing a part of the chlorine atoms of CFC with hydrogen atoms (hereinafter abbreviated as HCFC), fluorinated hydrocarbons, i.e., flons containing no chlorine atom (hereinafter abbreviated as HFC), alcohols and ketones, were listed in Detailed Description of this prior art to be combined unlimitedly with each other over very wide mixing range, and those exemplified are hydrocarbons such as propane and butane, HCFCs and HFCs such as 1,1-difluoro-1-chloroethane (hereinafter abbreviated as HCFC142b), 1,1-difluoroethane (hereinafter abbreviated as HFC152a) and 1,1,1,2-tetrafluoroethane (hereinafter abbreviated as HFC134a).
However, these substances differ from each other in the important factors determining the condition of a foam, such as specific parameters on styrene resin including permeability, saturated impregnation level and plasticizing effect, as well as, physical characteristics including critical temperature, critical pressure, vapor pressure and boiling point, the difference being prominent among alcohols, flons and hydrocarbons. Accordingly, when using dimethyl ether with these substances, a good foam which satisfies at the same time the performance and characteristics industrially required, such as compressive strength, heat insulating property, appearance, expansion ratio, closed cell ratio, manufacturing stability and the like in addition to the thickness and the sectional area, is not always obtained with mixing them just simply, but it is essential to select the kind of a substance to be mixed and to modify the manufacturing process appropriately in view of the use or purpose of foams to be obtained. With such a simple technical idea as just mixing them simply, it is obvious that the range of the mixing ratio over which industrially useful foams can be obtained is limited.
For example, 1,1-difluoroethane and ethanol are different from each other in various physical characteristics as shown below, and thus should be handled differently in view of the internal pressure and the cell forming ability.
1,1-Difluoroethane Ethanol Boiling point -24.1.degree. C. 78.32.degree. C. Critical temperature 113.3.degree. C. 243.1.degree. C. Critical pressure 46.1 Kgf/cm.sup.2 65.2 Kgf/cm.sup.2 Evaporation latent heat 79.4 Kcal/Kg 204 Kcal/Kg
However, the working examples in the prior art described above are limited to only use of dimethyl ether alone, the combinations of dimethyl ether with carbon dioxide, dimethyl ether with ethanol and dimethyl ether with ethanol plus carbon dioxide, and with respect to the suggested saturated hydrocarbons, HCFCs, HFCs and ketones, the using manner, the suitable range of mixing ratio and the characteristic properties or uses exhibited by using them were not disclosed specifically.
Furthermore, this prior art does not investigate sufficiently whether the foams disclosed actually satisfy other industrial requirements such as heat insulating property. That is, this prior art focuses only on getting a thick foam body with using, as a blowing agent, dimethyl ether which is expected to have cell-forming ability that means an ability of diffusing through cell membranes and forming cells (but the industrial requirement is, as a matter of course, not only to obtain thick foams but also to achieve physical characteristics such as heat insulating property and strength at the same time), and so the problems discussed above still remain unsettled.
On the other hand, as a substitute of CFCs, the use of HCFCs is proposed because of their lower adverse effect on the ozone layer and somewhat more preferable environmental compatibility. For example, JP, B, 57-7175 discloses use of HCFC142b as a blowing agent. Use of HFCs is also proposed. Since HFCs are believed generally to have no adverse effect on the ozone layer, they are considered to be more preferred than HCFCs in view of the environmental compatibility. For example, HFC134a was attempted to be used to form a foam as disclosed in JP, A, 1-98683, and a foam wherein 70% by weight or more of the amount of HFC134a used is allowed to remain in the cells thereof is disclosed in JP, A, 3-188137.
Nevertheless, alkyl chlorides represented by methyl chloride and ethyl chloride are the only kind of substances which are employed frequently in combination in the prior arts in spite of a large number of substances proposed as blowing agents, and the present situation where a substitute for alkyl chlorides is desired as described above still remains unchanged. In addition, the prior arts do not disclose any industrially useful information as to whether the substances can really provide, when used without being combined with an alkyl chloride, such industrially required physical characteristics as described in the prior arts, stability during manufacturing, production cost and the like.
Flons such as HCFCs and HFCs are considered to be useful in increasing the heat insulating ability of foams, but HCFCs pose a controversy on the effects on the ozone layer as described above, and HFCs intended to be used as a substitute of HCFCs are also desired to be reduced if possible in view of a recent trend of an environmental consciousness against the global warming although they are less affecting the ozone layer as compared with HCFCs or CFCs.
As discussed above, in spite of various technical attempts which have been made so far, there are no foams having an environmental compatibility together with industrial advantages such as high heat insulating property.
An object of the present invention is to provide a styrene resin extruded foam which is applicable to building materials, interior materials such as tatami floor sheet core, constructing materials and heat insulating materials, and in which an alkyl chloride, the use of which is desired to be reduced in view of an environmental compatibility, can be substituted, and which is excellent in terms of heat insulating property, compressive strength and extrusion stability during manufacturing.